One gallon of paint covers roughly 350 to 400 square feet per coat. That's the number to anchor every estimate on. Knowing it turns "how much paint do I need?" into simple arithmetic instead of a guess at the store.

The fastest way to turn that number into a shopping list is the paint calculator. Feed it your room dimensions and it handles the coverage math, coats, and rounding for you. But it helps to understand what's behind the number so you can judge when to lean toward the low end.

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Why 350-400 square feet?

Manufacturers print a coverage range because real-world conditions vary. The high end (around 400 square feet) assumes a smooth, sealed, primed surface and an experienced hand applying an even film. The low end (around 350 square feet) reflects ordinary walls and normal application.

When in doubt, estimate with 350. Buying slightly long is cheap insurance. Running out with one wall left is the expensive mistake, especially if your color was custom-tinted and the next can mixes a hair different.

What changes your real coverage

Several factors push your coverage toward the low end of the range:

  • Surface texture. Knockdown, orange peel, and stucco have more surface area than smooth drywall, so they drink paint. Heavy texture can cut coverage by 15 to 20 percent.
  • Porosity. Bare drywall, fresh patches, and unsealed surfaces soak up the first coat. Priming first preserves your topcoat coverage.
  • Color and hide. Low-hide colors like reds and bright yellows often need extra paint to look solid.
  • Application method. Spraying produces overspray and waste; brushing into detailed trim uses more than rolling a flat field.
  • Coats. This is the big one. Coverage is per coat, so two coats halves your effective square footage per gallon.

Doing the math

Here's the whole process in four steps:

  1. Find your wall area. Multiply the room's perimeter by the ceiling height. Our guide to calculating wall square footage walks through it, including subtracting doors and windows.
  2. Multiply by coats. Two coats is standard, so most projects double the area. See how many coats you actually need if you're unsure.
  3. Divide by coverage. Use 350 square feet per gallon to stay safe.
  4. Round up to whole gallons. Paint is sold by the can; you can't buy 1.3 gallons.

A worked example: a 12-by-15-foot room with 8-foot ceilings has a perimeter of 54 feet, giving 432 square feet of wall. Subtract a door (about 21 square feet) and two windows (about 15 square feet each), and you're at roughly 381 square feet of paintable wall. For two coats, that's 762 square feet. Divide by 350 and you get about 2.2 gallons. Round up to three gallons to be comfortable, or two gallons plus a quart if you measure carefully and your walls are smooth.

Notice how much the openings and the second coat move the number. If you'd estimated this room at one coat and ignored the door and windows, you'd have bought a single gallon and come up roughly a gallon and a half short. That's the gap that sends people back to the store mid-project. Building both coats and the subtractions into your estimate from the start is what helps keep a one-day paint job a one-day paint job.

Don't forget the ceiling. If you're painting it as well, add length times width as a separate figure and run it through the same coverage math. Ceilings are usually flat white and may cover in one coat over existing white, but a color change or a stained ceiling follows the same two-coat rule as the walls.

When a quart is the right buy

A quart covers about 100 square feet and earns its place when a full gallon would be overkill:

  • Touch-ups and small repairs after the main job
  • A single accent feature like a built-in, a door, or a closet interior
  • Trim and small details where you're brushing, not rolling

Keep a labeled quart of each color after a project. Future scuffs and nail holes are far easier to fix when you have a close match on the shelf.

Buy a little extra, on purpose

Coverage estimates assume ideal conditions you rarely get. A small buffer protects you from texture, low-hide colors, and the wall that turns out thirstier than expected. Rounding up to the next whole gallon usually builds in that cushion automatically.

Good tools also stretch every gallon. A 9-inch roller kit with the right nap for your wall texture transfers paint evenly instead of leaving thin, hungry patches that demand another pass.

The takeaway: 350-400 square feet per gallon per coat is your baseline. Estimate with the lower number, multiply by your coats, and round up. Measure twice. Buy once.